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Spotlight on Optics

March 2014

Spotlight Summary by Benjamin Eggleton

The demonstration of supercontinuum in all-silica photonic crystal fibers by Ranka et al. in 2000 was one of the most exciting breakthroughs in over a decade of photonics research and established a major new research direction in the field and partly lead to the Nobel Prize in Physics in 2005. The key to realizing supercontinuum generation was the strong mode confinement in the small core of the fiber and the massively engineered dispersion provided by the high index contrast. This demonstration stimulated research efforts around the world as research groups appreciated the potential for harnessing the properties of supercontinuum generation sources for wide ranging applications and also the more fundamental aspects of the supercontinuum generation dynamics. One of the key technical challenges that emerged was simply that the silica glass possessed only a modest optical nonlinearity so that very large peak intensities were required to initiate the supercontinuum generation process, and this in turn required large bulky lasers such as Ti-Sapphire lasers. It was clear that major breakthroughs were required to transform this breathtaking scientific advancement into a technological tool that could be used in everyday applications. At the same time, it was appreciated that the silica-fiber-based supercontinuum sources were limited in terms of the range of wavelengths that they could generate, primarily due to the absorptive properties of silica.

The key to solving these critical issues lies in the development of new optical materials, in which the nonlinear coefficient and transparency can be tailored and optimized. Nonlinear optical materials have been studied extensively for decades, since the invention of the laser itself. When considering supercontinuum generation in optical waveguides, which are based on amorphous glasses, the cubic nonlinearity is dominant. It turns out that there is a family of glasses, known as the chalcogenides that have been studied since the 1950's, that possess optical nonlinearities that are hundreds of times higher than that of silica and can be transparent over broad wavelength ranges. These glasses have therefore been of great interest for nonlinear optics since the mid 1990's when research at Bell Labs and Naval Research Labs initiated a major effort in this area, primarily motivated by fundamental challenges in optical communication networks that could be overcome by all-optical switching technologies. These glasses can be tailored by composition to possess excellent optical properties, with low propagation loss and absence of free carriers which limits semiconductors. Importantly they can be drawn into optical fibers and even tapered to create nanowires with sub-micron dimensions that then offer absolutely massive optical nonlinearities as high as 100,000 times that of standard SMF or more than 1000 times that of the photonic crystal fiber first used by Ranka. At the same time these fibers can possess desirable dispersion conditions because the high index contrast of the chalcogenide almost perfectly compensates the intrinsic material dispersion so that the net dispersion in the taper waist is optimal. Various groups have exploited this principle and have reported spectacular results of low threshold supercontinuum generation in the near infrared and even in the mid-infrared. Previous experiments reported by numerous groups have already achieved record levels of optical nonlinearity in dispersion engineered chalcogenide waveguides by tapering chalcogenide optical fibers to micron dimensions. The tapering systems are similar to the systems used widely in industry for tapering silica fibers except that the low melting temperature of the chalcogenide glass requires much lower temperatures to be used.

In the paper by Shabahang et al. the authors report very detailed characterization of chalcogenide nanotapers and compare experimental results to numerical simulations. The chalcogenide fibers used by these authors are surrounded by a thick thermoplastic polymer jacket which is retained during the tapering and gives the device superior mechanical robustness. The key advance in this paper is that the authors carefully characterize the nonlinearity of bulk chalcogenide samples using the well-known z-scan technique and then compare the nonlinearity with the chalcogenide nanotaper. The authors propagate pulses through the nanotapers and compare measurements of self-phase modulation to the calculated results based on the measured nonlinearities of the bulk measurements. They also observe supercontinuum in the nanotapers and show good agreement with calculated results. These demonstrations highlights that these chalcogenide nanowires can be robust and behave in a predictable and stable manner so they can be used in a wide range of important applications. They conclude that the tapering process does not modify the nonlinearity. From these and other results it is clear that chalcogenide devices offer enormous potential for nonlinear optical functionalities and can be robust and offer high performance.